1. Field of the Invention
This application generally relates to cement industry processes. In more specific aspects, the invention relates to methods of recovering products from cement kiln dust (CKD) produced during cement industry processes. The invention also relates to alternative disposal methods for waste sulfuric acid.
2. Description of the Prior Art
With the increased costs for disposing of wastes and the decrease of available places to send wastes, reducing the amount of wastes that need to be discarded has become more important. Non-hazardous wastes and hazardous wastes each have its own set of problems. Issues such as toxicity, harm to the environment, and the amount of waste created cause problems for those with these types of wastes that need to be discarded. For example, waste sulfuric acid is toxic and is regulated as a hazardous waste. The primary way to dispose of waste sulfuric acid is to have it incinerated. Incinerating waste, or spent, sulfuric acid is expensive and there are many regulatory requirements associated with its disposal.
Various methods have been used to produce or recover reusable compounds from waste materials, which in turn reduces the amount of waste that needs to be disposed of and decreases raw material costs for the reusable compound. Cement kiln dust (CKD) is one such type of waste that is produced during the use of a kiln during most cement manufacturing processes. Attempts have also been made to recover reusable compounds from the CKD and decrease the amount of CKD that has to be discarded.
One such example of making a reusable compound from waste materials can be found in U.S. Pat. No. 4,049,462, issued to Cocozza. The Cocozza patent relates to the chemical fixation of industrial desulfurization residues by forming a mixture of the residue, such as a flue gas desulfurization sludge, with an alkaline calcination stack dust, such as CKD in the presence of water. The pH of the mass is adjusted with sulfuric acid to a value of below about 7.0. The mixture is then dried into a shaped article to produce a solid, cement-like fixed product. Stack dust contains constituents such as calcium, silicon aluminum, iron, magnesium, sodium, potassium and associated constituents found in cement making and similar stack dust, for example, in the form of oxides and salts. The industrial waste residue can be any such residue or sludge as, for example, is produced in the after-removal of pollutants, such as sulfur constituents from the effluent or flue gas of basic manufacturing processes, such as fossil fuel consumption reaction or cement making or ore roasting reaction processes. These manufacturing processes often generate sulfur oxides, especially sulfur dioxide, which must be removed from the reaction effluent by scrubbing or absorbing techniques before venting to the atmosphere in order to avoid environmental pollution. An advantageous source of sulfuric acid is a spent industrial waste acid liquor, such as a spent pickle liquor of about 3-10% concentration.
Another example of trying to recover reusable products from CKD can be found in U.S. Pat. Nos. 4,716,027, and 4,915,914, both issued to Morrison. These patents describe neutralizing cement kiln dust so that it is suitable as a feed stock to the cement kiln and at the same time scrubbing the exhaust fumes to reduce SO2 levels. The precipitate from the neutralization process is suitable as kiln feed stock. The precipitate is sent to a kiln with other raw materials, where the raw materials are heated and produce clinker. The clinker from the kiln is then ground and mixed with gypsum to form cement. The alkali salt solution from the neutralization process is dried and forms a fertilizer. This process decreases the amount of waste that is produced due to use of a kiln during the cement manufacturing process.
Since gypsum is mixed with clinker to produce cement, it is desirable to recover gypsum from CKD. Gypsum can be produced by various method. One such method can be found in the Kikkawa patent, U.S. Pat. No. 5,788,944. The Kikkawa patent discloses a process where exhaust gas is brought into contact with liquid to absorb sulfur oxide. Limestone particles with specific diameters are retained in a zone for contact with absorbing liquid to neutralize the liquid. The gypsum formed thereby is drained and recycled.
In addition to gypsum, other materials that can be reused within the cement process. An example of this can be found in U.S. Pat. No. 6,231,767, issued to Krofchak. The Krofchak patent discloses a process for treating phosphatic clay suspensions, waste clay and phosphogypsum. This includes deflocculation to create a suspension of these components. The resulting phosphatic mineral and sand suspension is dissolved in dilute sulfuric acid to separate a phosphatic fraction as phosphoric acid from a sand fraction. A cementitious material is produced that is formed into an inert solid material.
A need exists for a process that will reduce the amount of CKD that needs to be disposed of as a result of existing cement processes and produce a product that is reusable within the cement production process. It is an object and a goal to convert a waste product to a desirable product, particularly one that currently is purchased to reduce raw material costs within the cement production process. Another object and goal of the present invention is to reduce the amount of gypsum that is purchased by recovering gypsum from waste streams from the kilns used within the cement manufacturing process. A further object and goal is to decrease the amount of cement kiln dust that needs to be disposed of and recovering a useful product from the cement kiln dust. It is yet another object and goal to provide a use for spent or waste sulfuric acid, as opposed to disposing of the spent or waste sulfuric acid.
In order to meet one or more of the identified objects, the present invention advantageously includes a method for treating raw or modified CKD using waste or by-product sulfuric acid solution to form a gypsum product or gypsum blend. Raw CKD, or primary CKD, contains the highest level of calcium carbonate on a weight basis. Modified CKD, or subsequent grades of CKD can contain much lower levels of calcium carbonate. The gypsum product can include calcium sulfate dihydrate (CaSO4.2H2O), anhydrite forms, co-products, and intermediates. The method disclosed can produce a Portland cement quality product. The sulfuric acid solution reacts with calcium carbonate and calcium oxide forming calcium sulfates. Other forms of sulfuric acid can be used, such as oleum or fuming sulfuric acid. The use of this gypsum blend in Portland cement manufacturing with clinker is functionally equivalent to that of commercially mined gypsum currently employed by most Portland cement manufacturers.
Other embodiments of the present invention are also provided, all of which are believed to improve the quality of the gypsum produced for use in Portland cement blending. As one of the alternate embodiments, the invention advantageously includes sweetening the raw or modified CKD with calcium rich by-products or limestone, shale or dolomite ores to enhance the calcium oxide equivalent available to the sulfuric acid for reaction. Another embodiment of this invention preferably includes the removal of alkali as an Na2O, or sodium oxide, equivalent in the raw or modified CKD or gypsum co-product of the reaction through removal of water-soluble components. In this embodiment, the treatment of the CKD-Sulfuric Acid product will improve the quality of the Portland cement grade calcium sulfate blend by lowering the potassium and sodium water-soluble components using pre-treatment or post-treatment methods described herein.
The invention also advantageously provides a method and apparatus for producing Portland cement by utilizing the gypsum product derived from the CKD and produced in accordance with the present invention.